Sulfur contaminants in fossil fuels can create problems in refinery processes which can be costly to rectify. The sulfur contaminants that occur in fossil fuels fall into either of the following general classes: mineralized (inorganic, e.g., pyritic) sulfur and organic sulfur (sulfur that is covalently bound to carbonaceous molecules, referred to as organosulfur compounds). The presence of sulfur has been correlated with corrosion of pipeline, pumping and refining equipment, and with premature breakdown of combustion engines. Sulfur also poisons many catalysts which are used in the refining of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products, such as sulfur dioxide, leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as on agricultural areas located downwind of combustion facilities. Monticello, D. J. and W. R. Finnerty, (1985) Ann. Rev. Microbiol. 39:371-389. Regulations such as the Clean Air Act of 1964 require the removal of sulfur, either pre- or post-combustion, from virtually all coal- and petroleum-based fuels. Conformity with such legislation has become increasingly problematic due to the rising need to utilize lower grade, higher-sulfur fossil fuels as clean-burning, low-sulfur petroleum reserves become depleted, as well as the progressive reductions in sulfur emissions required by regulatory authorities. Monticello, D. J. and J. J. Kilbane, "Practical Considerations in Biodesulfurization of Petroleum", IGT's 3d Intl. Symp. on Gas, Oil, Coal, and Env. Biotech., (Dec. 3-5, 1990) New Orleans, La.
One technique which is currently employed for the pre-combustion removal of organic sulfur from liquid fossil fuels, e.g., petroleum, is hydrodesulfurization (HDS). HDS is suitable for the desulfurization of fossil fuels wherein organosulfur compounds account for a significant, e.g., a major, proportion of all sulfur contaminants present. HDS is thus useful for treating crude oil or bitumen, petroleum distillate fractions or refining intermediates, liquid motor fuels, and the like. HDS is more particularly described in Shih, S. S. et al., "Deep Desulfurization of Distillate Components", Abstract No. 264B AIChE Chicago Annual Meeting, presented Nov. 12, 1990, (complete text available upon request from the American Institute of Chemical Engineers); Gary, J. H. and G. E. Handwerk, (1975) Petroleum Refining: Technology and Economics, Marcel Dekker, Inc., New York, pp. 114-120, and Speight, J. G., (1981) The Desulfurization of Heavy Oils and Residue, Marcel Dekker, Inc., New York, pp. 119-127. HDS is based on the reductive, conversion of organic sulfur into hydrogen sulfide (H.sub.2 S) in the presence of a metal catalyst. HDS is carried out under conditions of elevated temperature and pressure. The hydrogen sulfide produced as a result of HDS is a corrosive gaseous substance, which is stripped from the fossil fuel by known techniques. Elevated or persistent levels of hydrogen sulfide are known to poison (inactivate) the HDS catalyst, complicating the desulfurization of liquid fossil fuels that are high in sulfur.
Organic sulfur in both coal and petroleum fossil fuels is present in a myriad of compounds, some of which are termed labile in that they can readily be desulfurized, others of which are termed refractory in that they do not easily yield to conventional desulfurization treatment, e.g., by HDS. Shih, S.S. et al. Frequently, then, even HDS-treated fossil fuels must be post-combustively desulfurized using an apparatus such as a flue scrubber. Flue scrubbers are expensive to install and difficult to maintain, especially for small combustion facilities. Moreover, of the sulfur-generated problems noted above, the use of flue scrubbers in conjunction with HDS is directed to addressing environmental acid deposition, rather than other sulfur-associated problems, such as corrosion of machinery and poisoning of catalysts.
Recognizing these and other shortcomings of HDS, many investigators have pursued the development of microbial desulfurization (MDS). MDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. Thus, MDS typically involves mild (e.g., ambient or physiological) conditions, and does not involve the extremes of temperature and pressure required for HDS. It is also generally considered advantageous that biological desulfurizing agents can renew or replenish themselves under suitable conditions. Microbial desulfurization technology is reviewed in Monticello and Finnerty (1985), 39 Ann. Rev. Microbiol. 371-389 and Bhadra et al. (1987), 5 Biotech. Adv. 1-27. Hartdegan et al. (1984), 5 Chem. Eng. Progress 63-67 and Kilbane (1989), 7 Trends Biotechnol. (No. 4) 97-101 provide additional commentary on developments in the field.
Several investigators have reported mutagenizing naturally-occurring bacteria into mutant strains with the acquired capability of breaking down, i.e., catabolizing, dibenzothiophene (DBT). Hartdegan, F. J. et al., (May 1984) Chem. Eng. Progress 63-67. DBT is representative of the class of organic sulfur molecules found in fossil fuels from which it is most difficult to remove sulfur by HDS. Most of the reported mutant microorganisms act upon DBT nonspecifically, by cleaving carbon-carbon bonds, thereby releasing sulfur in the form of small organic breakdown products. One consequence of this microbial action is that the fuel value of a fossil fuel so treated is degraded. Isbister and Doyle, however, reported the derivation of a mutant strain of Pseudomonas which appeared to be capable of selectively liberating sulfur from DBT, thereby preserving the fuel value of treated fossil fuels. U.S. Pat. No. 4,562,156.
Kilbane recently reported the mutagenesis of a mixed bacterial culture, producing a bacterial consortium which appeared capable of selectively liberating sulfur from DBT by an oxidative pathway. Resour. Cons. Recycl. 3:69-79 (1990). A strain of Rhodococcus was subsequently isolated from the consortium. This strain, which has been deposited with the American Type Culture collection under the terms of the Budapest Treaty as ATCC No. 53968 and also referred to as IGTS8, is a source of biocatalytic activity as described herein. Microorganisms of the ATCC No. 53968 strain liberate sulfur from forms of organic sulfur known to be present in fossil fuels, including DBT, by the selective, oxidative cleavage of carbon-sulfur bonds in organic sulfur molecules. Kilbane has described the isolation and characteristics of this strain in detail in U.S. Pat. No. 5,104,801.